fix: Remove unsupported check constraint from user_ebook_folders table

- Remove complex CHECK constraint with subquery that PostgreSQL doesn't support
- Add comment explaining admin-only access is enforced at application level
- Update role system notes to clarify access control implementation
- Fix Bruno request failures due to missing database table

database/schema/schema.sql

@@ -68,10 +68,8 @@ CREATE TABLE user_ebook_folders (
     user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     folder_path VARCHAR(500) NOT NULL,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
-    UNIQUE(user_id, folder_path),
-    CONSTRAINT admin_only_folder CHECK (EXISTS (
-        SELECT 1 FROM users WHERE id = user_id AND role = 'admin'
-    ))
+    UNIQUE(user_id, folder_path)
+    -- Note: Admin-only access is enforced at application level in handlers/auth.go
 );
 
 -- Create indexes for better query performance
@@ -94,4 +92,5 @@ COMMENT ON COLUMN ebook_ratings.rating IS 'Rating scale 1-10 (odd numbers = half
 -- - Admin users can: add/edit/delete folders, scan ebooks, modify ebook metadata, delete ebooks
 -- - Regular users can: view all ebooks, rate ebooks, track reading progress, manage their profile
 -- - To create first admin: UPDATE users SET role = 'admin' WHERE email = 'your-admin-email';
--- - Only admins can manage folders and ebook metadata
+-- - Only admins can manage folders and ebook metadata (enforced at application level)
+-- - user_ebook_folders table admin-only access is enforced by AdminMiddleware in handlers